1. Field
The present application generally relates to optical metrology, and, more particularly, to optimizing the diffraction characteristics of wafer structures for optical metrology.
2. Related Art
In semiconductor manufacturing, periodic gratings are typically used for quality assurance. For example, one typical use of periodic gratings includes fabricating a periodic grating in proximity to the operating structure of a semiconductor chip. The periodic grating is then illuminated with an electromagnetic radiation. The electromagnetic radiation that deflects off of the periodic grating are collected as a diffraction signal. The diffraction signal is then analyzed to determine whether the periodic grating, and by extension whether the operating structure of the semiconductor chip, has been fabricated according to specifications.
In one conventional system, the diffraction signal collected from illuminating the periodic grating (the measured-diffraction signal) is compared to a library of simulated-diffraction signals. Each simulated-diffraction signal in the library is associated with a hypothetical profile. When a match is made between the measured-diffraction signal and one of the simulated-diffraction signals in the library, the hypothetical profile associated with the simulated-diffraction signal is presumed to represent the actual profile of the periodic grating.
The library of simulated-diffraction signals can be generated using a rigorous method, such as rigorous coupled wave analysis (RCWA). More particularly, in the diffraction modeling technique, a simulated-diffraction signal is calculated based, in part, on solving Maxwell's equations. Calculating the simulated diffraction signal involves performing a large number of complex calculations, which can be time consuming and costly.
At various points of the wafer processing, it is desirable to measure the structure before, during, and/or after a fabrication process. For example, after exposure of the resist and prior to stripping away the unexposed resist in photolithography, there is a need to measure the structure in order to predict if the exposure step was performed within the processing window for the recipe. The difficulty of making a successful measurement is that the exposed and unexposed resist have very similar optical properties. Another example is the need to measure the structure while development of a previously exposed resist is ongoing. The information about the structure profile obtained while development of the resist is in process can be used to control the resist development or adjust a process variable in the fabrication cluster.